This invention generally relates to welding systems, and is specifically concerned with a system and method for laser welding a sleeve to the inner surface of a heat exchanger tube in order to repair the tube.
Systems for laser welding sleeves to the inner surfaces of heat exchanger tubes are known in the prior art. Such systems are particularly useful in maintaining the integrity of the heat exchanger tubes used in nuclear steam generators. However, before either the utility or the limitations of such systems can be appreciated, some general background as to the structure, operation and maintenance of nuclear steam generators is necessary.
Nuclear steam generators are generally comprised of a bowl-shaped primary side, a tubesheet disposed over the top of the primary side, and a cylindrically shaped secondary side which in turn is disposed over the tubesheet. Hot, radioactive water from the reactor core circulates through the primary side of the steam generator, while non-radioactive water is introduced into the secondary side. The tubesheet hydraulically isolates but thermally connects the primary side to the secondary side by means of a number of U-shaped heat exchanger tubes whose bottom ends are mounted in the tubesheet. Hot, radioactive water from the primary side flows through the interior of these heat exchanger tubes while the exterior of these tubes comes into contact with the non-radioactive water in the secondary side in order to generate non-radioactive steam.
In the secondary side of such steam generators, the legs of the U-shaped heat exchanger tubes extend through bores present in a plurality of horizontally-oriented support plates that are vertically spaced from one another, while the ends of these tubes are mounted within bores located in the tubesheet. Small, annular spaces are present between these heat exchanger tubes and the bores in the support plates and the tubesheet which are known in the art as "crevice regions". Such crevice regions provide only a very limited flow path for the feed water that circulates throughout the secondary side of the steam generator, which may cause "dry boiling" to occur wherein the feed water boils so rapidly that these regions can actually dry out for brief periods of time before they are again immersed by the surrounding feed water. This chronic drying-out may cause impurities in the water to precipitate and collect in these crevice regions. These precipitates can ultimately create sludge and other debris that may promote the occurrence of stress corrosion cracking in the regions of the tubes surrounded by the bores of the tubesheet and the support plates which, if not repaired, will ultimately allow radioactive water from the primary side to contaminate the non-radioactive water in the secondary side of the generator.
To prevent such contamination from occurring, a repair procedure known as "sleeving" has been developed wherein a tubular sleeve formed from the same stainless steel as the damaged heat exchanger tube is slid up within the tube so that it traverses the corroded or otherwise damaged length of the tube. The ends of the sleeve are then affixed to the inner surfaces of the tubes in order to form a hydraulic "bridge" across the corroded or otherwise degraded length of the tube.
In the past, the ends of such sleeves have been affixed around the inner diameters of the heat exchanger tubes by mechanical expansions, brazing, and laser welding by means of a thin circle of laser light. While all three of these techniques have proven themselves to be effective in the field, laser welding is now thought to be the overall best technique for a variety of reasons. First, a properly-executed weld joint between the outer diameter of a sleeve and the inner diameter of a tube results in the strongest and most leak-proof connection between the tube and the sleeve. Secondly, such welding over only a thin circle around the inner diameter of the sleeve may cause the least amount of adverse metallurgical changes to occur in the Inconel.RTM. forming the tube and the sleeve. By contrast, mechanical expansions require a significant portion of both the tube and the sleeve to be radially and inelastically deformed, thereby work-hardening the metal. Brazing necessitates the application of a large amount of heat over a large length of the sleeve and tube, which may result in adverse changes in the grain structure of the metal in these regions that renders the metal more susceptible to stress corrosion cracking. Brazing may also cause thermal stresses to occur in the tubes as a result of thermal expansion. While there are procedures which satisfactorily relieve such thermally-induced stresses, the use of such stress-relief procedures protracts the time necessary to install the sleeves which is undesirable, since for many utilities the revenue losses associated with any repair procedure exceed over a million dollars per day.
Laser welding is typically implemented by a welding system comprising an elongated weld head that is insertable within a sleeve and that is concentrically disposed around the inner walls of a tube to be repaired. Laser light is typically transmitted to the weld head through a fiber optic cable. The weld head may include a prism or mirror for radially directing a focused laser beam onto the inner wall of the sleeve, and the system may include means for rotating the mirror or prism of the weld head around the inner diameter of the sleeve to trace a circular path around the sleeve. An example of such a laser welding system is disclosed in U.S. Pat. No. 4,839,495. Because laser welding techniques are capable of reliably creating a weld joint by the application of a radially-directed beam of laser light along a very thin circle around the inner diameter of the sleeve, adverse changes in the metals forming the sleeve in the tube are minimized to very small localities in the tube. Moreover, the highly localized nature of the heat used to weld the sleeve to the tube minimizes thermal differential expansion, thereby obviating the need for time consuming stress-relief techniques. These advantages, in combination with the strength, durability, and leak-proof seal that such welding provides, makes laser welding the most desirable choice for affixing a sleeve to the inner surfaces of damaged heat exchanger tubes.
Unfortunately, none of the prior art laser welding systems that the applicant is aware of is capable of effectively and reliably welding the inner wall of a sleeve or a tube that is significantly out-of-round. While prior art systems are known which include spring-loaded centering devices that keep the rotating housing of the weld head aligned with the center line of the tube or sleeve, the applicant has noted that the radially-directed laser beam may well be thrown out of focus as the beam sweeps around the tube or sleeve due to the varying distances between the mirror or prism that radially directs the laser beam, and the tube or sleeve wall. The resulting out-of-focus laser beam is not as capable of reliably welding a tube or sleeve as a beam that stays in constant sharp focus.
Still another problem that the applicants have observed in prior art systems stems from the use of two or more S-shaped bends in the distal end of the fiber-optic cable disposed in the tubular housing of the weld head. Such bending of the cable is sometimes necessitated by the presence of large components in the proximal ends of the housing, such as the electric motors and drive train used to rotate the mirrors or prisms which radially steer the laser beam around the inner wall of the tube or sleeve. However, the applicant has noted that when such fiber-optic cables traverse such large components by the use of two S bends, the resulting transmissivity of the cable is seriously impaired. This problem may be remedied by lengthening the housing so as to provide a greater distance between the S bend above the electric motor or other component, and the S bend below the motor. However, the long length of the weld head housing which results from such a solution makes it difficult, if not impossible, to insert and withdraw the weld head into heat exchanger tubes arranged around the periphery of the tubesheet due to the proximity of these tubes to the inwardly curving walls of the primary side of the generator.
Still another shortcoming associated with the prior art has been the difficulty, if not the impossibility, of replacing the mirror or prism which radially deflects and steers the laser beam around the inner wall of the tube or sleeve. Despite the provision of a constant stream of inert gas through the welding housing during the welding operation, the resulting fusion and vaporization of metal ultimately fouls the mirror or prism to an extent which renders it useless. The inability to easily replace such a fouled mirror necessitates replacement of the entire weld head, which is a time consuming and expensive operation. Additionally, such weld head replacement slows down the entire maintenance operation, and necessitates that more than one intact welding head be kept on hand at all times during a maintenance operation.
Clearly, what is needed is a laser welding system that is capable of accurately and reliably welding sleeves or tubes which may be slightly out-of-round. Preferably, the housing of such a welding system would be short enough so peripheral tubes could be easily serviced. Finally, it would be desirable if the mirror or prism used in the beam deflecting mechanism could be easily and expeditiously replaced in the field without the need for maintaining more than one complete welding head on hand at all times.